1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image reading device to read image data from an original document placed on a document stand by scanning it using a carriage driven by a pulse motor, an image reading method, and an image forming apparatus including the image reading device.
2. Description of the Background
There is increasing demand for a shorter wake-up time from a standby mode to an active mode as well as higher processing speed for image reading devices (such as scanners) included in image forming apparatuses such as copiers and digital multifunction printers.
When not in operation, the image reading device is generally either in a power-off mode, in which all power supply is turned off, or in an energy-saving mode, in which power is only partially supplied. Specifically, the image reading device enters the power-off mode when a main power source is turned off to shut down the image reading device. By contrast, the image reading device enters the energy-saving mode when a next operation is not performed for a certain period of time after completion of the previous operation. During the energy-saving mode, minimum power is supplied to a minimum number of units to operate a control circuit, and no power is supplied to the rest of the units. A shorter wake-up time is especially required when the image reading device is returned to the active mode from the energy-saving mode. How this shorter wake-up time is accomplished requires a detailed discussion of the structure and operation of a typical image reading device, provided below.
The image reading device typically includes a lamp, a reflector, a first carriage including a first mirror, a second carriage including second and third mirrors, a home position sensor to detect whether or not the first carriage is positioned at a home position, a contact glass on which an original document is to be placed, a reference whiteboard used for generating shading data and adjusting a reference level, optical elements such as a lens and a CCD, a motor for moving the first and second carriages, and so forth. The first and second carriages are moved in a sub-scanning direction to read data on the reference whiteboard and the original document. When not scanning, the first and second carriages stay at the home position.
FIG. 1A is a top view illustrating a related-art image reading device. FIG. 1B is a front view illustrating the related-art image reading device illustrated in FIG. 1A in which first and second carriages are positioned at a home position. FIG. 1C is a view illustrating a detection result output from a home position sensor in which a sub-scanning direction of the carriages is represented by a horizontal axis.
Referring to FIGS. 1A and 1B, an image reading device 1 includes a document stand 2, a contact glass, not shown, provided to the document stand 2, and a reference whiteboard 3 provided on a bottom surface of the document stand 2. The reference whiteboard 3 is provided upstream from the contact glass relative to a direction of movement of a carriage assembly. The carriage assembly includes a first carriage 4 and a second carriage 5. The first carriage 4 includes a lamp 6 serving as an illumination light source, and a first mirror 7 to reflect light reflected from an original document. The second carriage 5 includes second and third mirrors 8 and 9 to direct the light reflected from the first mirror 7 to an imaging lens 10. The first and second carriages 4 and 5 are moved at a speed ratio of 2 to 1, and a distance from the first and second carriages 4 and 5 to a CCD 11 is set to be constant. The CCD 11 reads an image of the original document, and is provided on a sensor board 12. The image of the original document formed by the imaging lens 10 is focused on an imaging surface of the CCD 11 and read by the CCD 11. At this time, data on the reference whiteboard 3 and the original document are read as image data. The first and second carriages 4 and 5 are positioned at a home position 13 when not scanning. The home position 13 is located a certain distance apart from where a home position sensor 14 is switched from OFF to ON when detecting that the first carriage 4 passes the home position sensor 14.
It should be noted that the lamp 6, the first mirror 7, and the second and third mirrors 8 and 9 hereinafter serve as optical elements. The first and second carriages 4 and 5 are moved by a driving force transmission mechanism, not shown, including a pulse motor.
FIG. 2 is a flowchart illustrating a process to return the image reading device 1 from the power-off mode or the energy-saving mode to the active mode. In the image reading device 1 of the related-art, the same process is performed to return the image reading device 1 from the power-off mode and the energy-saving mode to the active mode. Specifically, at S401, integrated circuits (ICs) are initialized to return the image reading device 1 from the power-off mode or the energy-saving mode to the active mode. At S402, the lamp 6 is turned on, and homing is performed at S403. It should be noted that the term “homing” as used in this specification includes relocation of the first carriage 4 at the home position 13. Homing includes two different processes depending on a state of the home position sensor 14 before homing, as described below.
In a case in which the home position sensor 14 is switched ON before homing, the first carriage 4 is moved to a position where the home position sensor 14 is switched OFF. Subsequently, a direction of movement of the first carriage 4 is reversed when the home position sensor 14 is switched OFF, and moved until the home position sensor 14 is switched ON. A pulse is counted from the position where the home position sensor 14 is switched ON from OFF to place the carriage 4 at the home position 13.
By contrast, in a case in which the home position sensor 14 is switched OFF before homing, the first carriage 4 is moved in a direction such that the home position sensor 14 is switched ON. A pulse is counted from the position where the home position sensor 14 is switched ON from OFF to place the carriage 4 at the home position 13.
After homing, at S404, the first carriage 4 is moved under the reference whiteboard 3 from the home position 13 to start initialization. At S405, initialization is performed to initialize the ICs and adjust gain and offset values. At S406, the adjusted gain value and a black level are confirmed and the first carriage 4 is moved to the home position 13. At S407, the lamp 6 is turned off, and the image reading device 1 is now in the active mode and ready to start reading images.
In the above-described image reading device 1, the motor for driving the first carriage 4 is not activated during the power-off mode and the energy-saving mode. Consequently, the first carriage 4 may be moved from the home position 13 due to vibration applied while, for example, in transit. As a result, homing is required to be performed to move the first carriage 4 to the home position 13 when the image reading device 1 is returned to the active mode. However, because a moving speed of the first carriage 4 during homing is constant and slow, a longer time is required to complete homing.
Generally speaking, because a main power source is turned off, the image reading device 1 does not enter the energy-saving mode while in transit. Therefore, it is assumed that the first carriage 4 does not move much from the home position 13 during the energy-saving mode due to vibration. However, during the energy-saving mode, the first carriage 4 is likely to be positioned where it is positioned immediately before the image reading device 1 enters the energy-saving mode. Consequently, homing is still required to be performed when the image reading device 1 is returned from the energy-saving mode to the active mode.
Although techniques to reduce the time required to restart the image reading device from shut down have been disclosed, those techniques are not effective to reduce the time required to return the image reading device from the energy-saving mode to the active mode.